1. Field of the Invention
The present invention relates to a method and system for a floating structure using a marine riser while drilling. In particular, the present invention relates to a method and system for return of drilling fluid from a sealed marine riser to a floating structure while drilling in the floor of an ocean using a rotatable tubular.
2. Description of the Related Art
Marine risers extending from a wellhead fixed on the floor of an ocean have been used to circulate drilling fluid back to a floating structure or rig. The riser must be large enough in internal diameter to accommodate the largest bit and pipe that will be used in drilling a borehole into the floor of the ocean. Conventional risers now have internal diameters of approximately 20 inches, though other diameters are and can be used.
An example of a marine riser and some of the associated drilling components, such as shown in FIG. 1, is proposed in U.S. Pat. No. 4,626,135, assigned on its face to Hydril Company, which is incorporated herein by reference for all purposes. Since the riser R is fixedly connected between the floating structure or rig S and the wellhead W, as proposed in the '135 patent, a conventional slip or telescopic joint SJ, comprising an outer barrel OB and an inner barrel IB with a pressure seal therebetween, is used to compensate for the relative vertical movement or heave between the floating rig and the fixed riser. Diverters D have been connected between the top inner barrel IB of the slip joint SJ and the floating structure or rig S to control gas accumulations in the subsea riser R or low pressure formation gas from venting to the rig floor F.
One proposed diverter system is the TYPE KFDS diverter system, previously available from Hughes Offshore, a division of Hughes Tool Company, for use with a floating rig. The KFDS system's support housing SH, shown in FIG. 1A, is proposed to be permanently attached to the vertical rotary beams B between two levels of the rig and to have a full opening to the rotary table RT on the level above the support housing SH. A conventional rotary table on a floating drilling rig is approximately 49½ inches in diameter. The entire riser, including an integral choke line CL and kill line KL, are proposed to be run-through the KFDS support housing. The support housing SH is proposed to provide a landing seat and lockdown for a diverter D, such as a REGAN diverter also supplied by Hughes Offshore. The diverter D includes a rigid diverter lines DL extending radially outwardly from the side of the diverter housing to communicate drilling fluid or mud from the riser R to a choke manifold CM, shale shaker SS or other drilling fluid receiving device. Above the diverter D is the rigid flowline RF, shown configured to communicate with the mud pit MP in FIG. 1, the rigid flowline RF has been configured to discharge into the shale shakers SS or other desired fluid receiving devices. If the drilling fluid is open to atmospheric pressure at the bell-nipple in the rig floor F, the desired drilling fluid receiving device must be limited by an equal height or level on the structure S or, if desired, pumped by a pump up to a higher level. While the choke manifold CM, separator MB, shale shaker SS and mud pits MP are shown schematically in FIG. 1, if a bell-nipple is at the rig floor F level and the mud return system is under minimal operating pressure, these fluid receiving devices may have to be located at a level below the rig floor F for proper operation. Hughes Offshore has also provided a ball joint BJ between the diverter D and the riser R to compensate for other relative movement (horizontal and rotational) or pitch and roll of the floating structure S and the fixed riser R.
Because both the slip joint and the ball joint require the use of sliding pressure seals, these joints need to be monitored for proper seal pressure and wear. If the joints need replacement, significant rig down-time can be expected. Also, the seal pressure rating for these joints may be exceeded by emerging and existing drilling techniques that require surface pressure in the riser mud return system, such as in underbalanced operations comprising drilling, completions and workovers, gas-liquid mud systems and pressurized mud handling systems. Both the open bell-nipple and seals in the slip and ball joints create environmental issues of potential leaks of fluid.
Returning to FIG. 1, the conventional flexible choke line CL has been configured to communicate with a choke manifold CM. The drilling fluid then can flow from the manifold CM to a mud-gas buster or separator MB and a flare line (not shown). The drilling fluid can then be discharged to a shale shaker SS to mud pits and pumps MP. In addition to a choke line CL and kill line KL, a booster line BL can be used. An example of some of the flexible conduits now being used with floating rigs are cement lines, vibrator lines, choke and kill lines, test lines, rotary lines and acid lines.
Therefore, a floating rig mud return system that could replace the conventional slip and ball joints, diverter and bell-nipple with a seal below the rig floor between the riser and rotating tubular would be desirable. More particularly it would be desirable to have a seal housing, that moves independent of the floating rig or structure but with the rotatable tubular to reduce vertical movement between the rotating seal and tubular, that includes a flexible conduit or flowline from the seal housing to the floating structure to compensate for resulting relative movement of the structure and the seal housing. Furthermore, it would be desirable if the seal between the riser and the rotating tubular would be accessible for ease in inspection, maintenance and for quick change-out.